The following was originally written as an email, but might form the basis of a photography basics article if anyone fancies adding to it. Needs a bit more intro - as is it's a bit like joining a conversation half-way through.

A.

There are two parts to getting the right exposure - the shutter speed, and the aperture size, and they're closely related.

SHUTTER SPEED

The shutter speed is mostly self explanatory: the faster the shutter speed, the shorter the exposure and thus less light reaches the film / sensor, and vice versa.

Modern cameras can do all sorts of weird and wonderful shutter speeds, but the "main" ones on the scale include speeds like 1/30th, 1/60th, 1/125th, 1/250th etc. Note that each speed is half of the previous one - this is a key point which we'll come back to in a bit...

For many kinds of picture-making the shutter speed is almost academic (landscapes for example); the aperture is more important so you might choose an aperture and let the camera choose whatever shutter is needed to get the right exposure.

Other times you'll want a shutter speed to achieve a certain effect - to freeze action in a wildlife/sports shot, or to get misty blurry water in a fast flowing brook.

Something important to keep in mind when you're hand-holding the camera is that to avoid camera shake and blurry pictures the shutter speed has to be the inverse of the focal length. So if you're using a 250mm lens, you'll need 1/250th or faster to achieve sharp pics. Note that this is the old rule that applied to film cameras - it's slightly more complicated with digital but I'll not burden you with that for now. Just try and be mindful of this inverse rule when out in the field. Obviously, using a tripod dispels this concern completely.

APERTURE SIZE

All lenses have a set of blades inside them that effectively form a light barrier with a hole in the middle. This hole can be increased in size to let more light in, or decreased to reduce the light. The hole is the "aperture size", and we'll look at why you'd want to vary this in a bit.

The aperture size is expressed as number such as f2.8, f4, f11 etc. The 'f' bit stands for 'focal length ratio'; don't worry about the maths, just keep in mind that a SMALLER f-number means a BIGGER hole (i.e. more light), and a BIGGER f-number means a SMALLER hole (less light).

On old cameras you'd turn a ring on the lens to change the aperture size, and each position was a definite mechanical click on the ring. Hence the term "f-stop", because when you turned the ring you'd hear and feel a definite mechanical stop when you engaged the next aperture size. The phrase "stop" has come to have a wider meaning, which we'll return to in a moment.

Like shutter speeds, modern cameras can do all sorts of weird and wonderful aperture sizes, but the "main" ones on the scale include f2.8, f4, f5.6, f8, f11, f16. These are the crucial ones because each one on the scale is either exactly double, or exactly, half, of its predecessor.

So for example, f2.8 lets in twice the amount of light as f4. Meanwhile f5.6 lets in half the amount of light as f4. This is key, as we'll now see...

SHUTTER SPEED AND APERTURE SIZE TOGETHER

Recall how the main scale of shutter speeds sees the light halved as the speed increases - 1/30th, 1/60th etc. So in terms of calculating exposure, there's a direct relationship between shutter speed and aperture size.

Say you're snapping some wildlife and the camera is suggesting 1/60th at f8. But because you're using the 250mm telephone lens, you ought to be using 1/250th not 1/60th to ensure sharp hand-held pics.

A shutter speed of 1/250th is "two stops" up the shutter speed scale - you've gone from 1/60th to 1/125th, then from 1/125th to 1/250th. You have halved the light intake, twice, thus reducing the light by a factor of 4. To compensate, you need to increase the aperture by "two stops" - to f4. Increasing from f8 to f5.6 doubles the light, increasing from f5.6 to f4 doubles it again, so you've redressed the balance of that factor of 4.

So in other words, 1/60th at f8, and 1/250th at f4, let in exactly the same amount of light, but the effects on your composition will be different.

Note how the term "stop" has come to mean a general purpose doubling or halving of light. If you increase exposure by "one stop", you're either halving the shutter speed, or doubling the aperture. If you decrease exposure by one stop, your either doubling the shutter speed, or halving the aperture.

In camera specs you'll also see the term EV or "exposure value". This means exactly the same as "stop" - so increasing exposure by 1EV doubles the light intake.

AND FINALLY - WHY YOU'D MANIPULATE THE APERTURE SIZE

The aperture size has a direct effect on your picture's depth of field - the zone of your pic that is in sharp focus. For a wildlife shot you'd probably want that squirrel razor sharp, but the foliage behind him all blurry and out of focus. For a landscape, you'd probably want the whole picture in sharp focus. Get this wrong and the picture probably won't look right; a sharp foliage backdrop to the squirrel would be distracting, whilst a landscape with part of the scene looking fuzzy would as a general rule look odd.

The primary tool for controlling depth of field is the aperture size: as aperture increases depth of field decreases, and vice versa. So for that squirrel you might want to focus on his eyes, set aperture to the widest possible (i.e. smallest f-number - probably f3.5 on your camera), keeping an eye on the shutter speed for camera-shake (and the wider aperture will help you to achieve a faster shutter speed). Meanwhile for that landscape you'd probably go for a minimum of f11, more like f16; shutter speed academic but you might need a tripod to avoid camera shake (especially as the smaller aperture will force a slower shutter speed).

Experience will tell you which aperture sizes work in different circumstances, and which aperture sizes produce roughly what sort of depth of field.

One last thing to bear in mind. The lens focal length also has a key impact on depth of field: wide-angle lenses produce a wide depth of field, telephoto lens compress the depth of field. Again, practice and experience will tell you what combinations of focal length and aperture size will yield what sort of results. It is possible to calculate the depth of field to the millimetre for a given set of circumstances - if you get into still life pics I'll tell you how to do it (the maths aren't difficult, but would be tedious out in the field).